Physiological differences in circadian rhythmicity have been reported between individuals with marked circadian chronotype--Morning (M) types, "early birds," and Evening (E) types, "night owls." E Types lag M types in the circadian peak time of some physiological variables, e.g., body temperature, heart rate, and blood presure. We do not know if M and E types vary by chronotype in some basic metabolic and stress- related functions. The aim of this exploratory study is to describe patterns of neuroendocrine hormones, blood pressure, heart rate, and body temperature over 38 hours in a purposive sample of healthy adults (12-M, 12-E). Chronotype will be quantified by a Morningness- Eveningness questionnaire and validated by continuous rectal body temperature measure.